tests/tracing: use a base trace type YAML file

tests/tracing: use a base trace type YAML file

This patch adds `tests/tracing/support/base.yaml` as a base YAML trace
type object for test configuration files to include.

`tests/tracing/conftest.py` is changed to add `tests/tracing/support` to
the barectf inclusion search directories and to only consider YAML files
in `tests/tracing/configs/succeed` (`tests/tracing/support/base.yaml` is
not a test).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>